As is well known, thermal secondary recovery operations are routinely employed to recover heavy hydrocarbons, e.g. heavy oil, from subterranean reservoirs (e.g. oil sands). Due to its high viscosity, the heavy oil must be heated in place to reduce its viscosity so it will flow from the reservoir. Probably the most common of such thermal recovery operations involves "steam stimulation" wherein the heavy oil is heated in place by steam which is injected into the reservoir. A steam stimulation or steamflood process can be carried out by either (a) injecting the steam into an injection well and then producing the hydrocarbons from a separate well or (b) injecting the steam and then producing the fluids through the same well.
In a typical, conventional gravity-dominated steamflood recovery operation, steam is injected into one well while formation fluids (e.g. oil) are produced through spaced production wells. These production wells normally have substantially vertical wellbores which are cased to at least a depth which lies adjacent the top of the oil sand. The lower end of the wellbore is then completed with a gravel pack or the like through the production interval.
Steam is injected through the injector well for an initial period (e.g. 3 to 24 months) in order to establish thermal communication between the injector well and the production wells. During this initial injection period, each production well may either produce cold oil at a low flow rate or be stimulated by cyclically injecting steam into the producing well, itself. Higher production flow rates normally occur only after thermal communication between wells has been established.
In a steam stimulation operation such as described above, steam is injected down the injection well and out into the formation. Due to its relative density, the steam tends to rise towards the top of the formation during injection. This natural gravity segregation results in the creation of a "steam chest" across the top of the producing formation which, in turn, results in early steam breakthrough and less than 100% vertical sweep of steam through the formation.
This is especially true where a production well is completed at the top of an oil sand where steam, upon breakthrough, will be produced into the wellbore and up through the annulus of the producing well. This results in a substantial loss of valuable steam and at the same time, may create severe back pressure and pump problems which seriously inhibit the production of oil from the reservoir.
In steamfloods of this type, it has been observed that high oil production rates usually occur within a 1 to 3 month period just prior to steam breakthrough at a production well. In an effort to delay steam breakthrough and thereby contain the steam within the reservoir for a longer period, the production wells are often cased to an extended depth lying well within the reservoir thereby isolating the upper portion of the reservoir behind the casing. While delaying steam breakthrough, unfortunately, the extended casing may also delay the production of hot oil since the steam chest will now be located a significant vertical distance above any openings in the casings and/or liner thereby allowing only cold oil to enter the well.
Other techniques have been proposed for improving the production of heavy oil from a reservoir by improving the sweep efficiency of the injected steam through the reservoir. One such technique involves the injection of a foam or other flow-blocking material into a formation to fill previous swept and/or more permeable zones of the reservoir before injecting the steam. Another technique involves the drilling of horizontal wells into the reservoir to intersect natural fracture systems of the reservoir and to provide a long completion interval within the reservoir. The present invention provides still another method for producing heavy hydrocarbons from a reservoir which use "inverted" production wells which, in turn, provide several apparent advantages over either vertical or horizontal production wells.